The present invention relates to a method of determining local structures in optical materials, especially crystals, and, more particularly, to a method of determining local structures in materials for microlithography, e.g. from calcium fluoride, and to the optical elements obtained from them.
For optical applications both in glasses and also in crystals the optical properties are specified in great detail besides the form or shape of the particular product. In addition these optical properties in glasses and crystals include properties, such as transmission, homogeneity of the index of refraction and interior quality, which can be damaged by bubbles and inclusions or schlieren. In crystals the interior quality is described by the so-called real structures, including point defects, dislocations and grain boundaries, especially small-angle grain boundaries. These defects or imperfections are frequently very much spatially located, in contrast to similar defects in glass. However they have a very strong influence on the physical effects connected with them, such as absorption properties, homogeneity of the index of refraction and double refraction or birefringence.
The use of crystals for optical components is continuously increasing especially because of the increasing use of wavelengths, which are outside the visible range and which are not transmitted through glass. There is thus an increasing demand for monocrystalline materials made from alkali metal fluorides and alkaline earth fluorides (CaF2, BaF2, SrF2 and so forth) for UV applications, such as UV lithography or lenses and windows for irradiating units and imaging units. The same is true for crystals in the infrared spectral region, which are required for many optical elements.
Particularly calcium fluoride single crystals (CaF2) are required as the starting material for optical components in DUV photolithography (DUV=deep UV) at wavelengths around or under 200 nm, especially at wavelengths of 248 nm, 193 nm and 157 nm, of the Excimer laser. These optical components are usually lenses, prisms and plates in the so-called steppers or excimer lasers. They serve particularly to optically form the fine structured integrated circuits on photolacquer coated semiconductor disks and/or with the masks used in photolithography.
The semiconductor disks are always increasing in size. Currently the semiconductor disks are already 250 mm. On the other hand, the semiconductor structures are always getting. Structure widths of 250 nm are already standard. Thus the optical components and/or the materials for them must have the required quality over their entire volume (for example round disks with about 250 nm diameter and about 50 mm thickness), also, on a small-scale.
Thus CaF2 crystals for applications in projection optics for the UV and DUV range must be free of coarse or gross defects, such as grain boundaries. For optical applications only crystals, which are single crystals over their entire volume, can be considered. A single crystal has primarily a desired orientation determined by a certain application, e.g. a crystallographic orientation <111>, <100> and <110>. Whether this single crystal is usable primarily for the desired optical purpose and for which application is determined by the local structures, such as disturbances and crystal defects, in the single crystalline region.
Single crystals are characterized by a long-range periodic crystal structure in bulk and thus a priori have a higher uniformity or homogeneity of physical properties. However since all parameters, especially heat flow, are not completely under control during the making of a single crystal in the crystal growth apparatus, the so-called real structure with standard defects forms during crystallization. Thus, for example, regions with birefringence and blocks with crystal orientations that are slightly different from each other arise. The orientation variations are within an angular range of about 1° to 5°. These local real structures limit the quality of the crystal in practice. Thus they may not exceed certain limiting values when such a crystal is used in a highly accurate optical system, such as a projection optic system for a stepper, which operates at 248 nm, 193 nm or 157 nm.
The local structures occurring in the above-described single crystals for optical applications are, above all, accumulations of defect locations, dislocations, glide planes and small-angle grain boundaries. Dislocations are statistically distributed defects and glide planes or glide strips are planar defects. Small-angle grain boundaries are boundary surfaces, which separate crystal regions with comparatively slight orientation differences (less than 10 degrees). Impurities frequently collect at small-angle grain boundaries, which are for example CaO deposits in the case of CaF2.
Small-scale structures only play a very subordinate role in the characterization of optical glasses. The quality of optical glasses is determined essentially by long-range inhomogeneities. Accordingly only gross or large-scale defects or faults are measured. The measurements have only a comparatively low spatial resolution.
When measurement methods commonly used for glass are transferred to or used for crystals, typical crystal defects are missed or not determined. As a result of that experiments that measure the quality of the starting crystals often produce good results. The crystals are judged acceptable for a wide range of applications. However often when optical components are built from the crystals, they do not meet the required specifications. Since small-scale defects arise also in glasses and other optical—also non-crystalline—materials, which interfere in optical systems with comparatively higher resolution, they may not be detected by the above-described methods.
These local small-scale defects in the structure of a “single-crystal” are thus decisive for the use of a crystal, for example, in projection optics for microlithography in the UV region and in the far UV region, which means wavelengths less than 200 nm.